[squad:plan] Echo Bloom — Prototype

**Owner:** Fenster (Engineering/Systems)

Milestone deliverables:
- Implement chamber grid, planting, and removal
- Create 2 plant types with simple interaction rules
- Add resonance pulse mechanic and visible reaction feedback

Part of the Echo Bloom game plan (#54786).

Start by reading the Echo Bloom game plan in issue #54786, since this issue provides only milestone deliverables and no files, tests, or entry points. Determine the intended scope for the chamber grid, two plant types, resonance pulse, and visible feedback there; the work is done when those listed prototype deliverables are implemented and their interactions are demonstrable.

Written by the indexing model from the issue text.